1. Field
The present invention relates to communications. More particularly, the present invention relates to a novel and improved method and apparatus for controlling transmission energy in a wireless communication system.
2. Background
The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of system users are present. Other multiple access communication system techniques, such as time division multiple access (TDMA) and frequency division multiple access (FDMA) are known in the art. However, the spread spectrum modulation technique of CDMA has significant advantages over these modulation techniques for multiple access communication systems. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled, “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,” assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, entitled, “SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,” assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein.
CDMA by its inherent nature of being a wideband signal offers a form of frequency diversity by spreading the signal energy over a wide bandwidth. Therefore, frequency selective fading affects only a small part of the CDMA signal bandwidth. Space or path diversity is obtained by providing multiple signal paths through simultaneous links from a mobile user through two or more cell-sites. Furthermore, path diversity may be obtained by exploiting the multipath environment through spread spectrum processing by allowing a signal arriving with different propagation delays to be received and processed separately. Examples of path diversity are illustrated in U.S. Pat. No. 5,101,501 entitled, “METHOD AND SYSTEM FOR PROVIDING A SOFT HANDOFF IN COMMUNICATIONS IN A CDMA CELLULAR TELEPHONE SYSTEM,” and U.S. Pat. No. 5,109,390 entitled, “DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM,” both assigned to the assignee of the present invention and incorporated by reference herein.
A method for transmission of speech in digital communication systems that offers particular advantages in increasing capacity while maintaining high quality of perceived speech is by the use of variable rate speech encoding. The method and apparatus of a particularly useful variable rate speech encoder is described in detail in U.S. Pat. No. 5,414,796, entitled, “VARIABLE RATE VOCODER,” assigned to the assignee of the present invention and incorporated by reference herein.
The use of a variable rate speech encoder provides for data frames of maximum speech data capacity when said speech encoding is providing speech data at a maximum rate. When a variable rate speech coder is providing speech data at a less that maximum rate, there is excess capacity in the transmission frames. A method for transmitting additional data in transmission frames of a fixed predetermined size, wherein the source of the data for the data frames is providing the data at a variable rate is described in detail in U.S. Pat. No. 5,504,773, entitled, “METHOD AND APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSION,” assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein. In the above mentioned patent application a method and apparatus is disclosed for combining data of differing types from different sources in a data frame for transmission.
In frames containing less data than a predetermined capacity, power consumption may be lessened by transmission gating a transmission amplifier such that only parts of the frame containing data are transmitted. Furthermore, message collisions in a communication system may be reduced if the data is placed into frames in accordance with a predetermined pseudorandom process. A method and apparatus for gating the transmission and for positioning the data in the frames is disclosed in U.S. Pat. No. 5,659,569, entitled, “DATA BURST RANDOMIZER,” assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein.
A useful method of power control of a mobile in a communication system is to monitor the power of the received signal from the mobile station at a base station. The base station in response to the monitored power level transmits power control bits to the mobile station at regular intervals. A method and apparatus for controlling transmission power in this fashion is disclosed in U.S. Pat. No. 5,056,109, entitled, “METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM,” assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein.
In a communication system that provides data using a QPSK modulation format, information regarding the transmitted data signal can be obtained by taking the cross product of the I and Q components of the QPSK signal with the estimate of the communications channel. By knowing the relative phases of the two components, one can determine roughly the velocity of the mobile station in relation to the base station. A description of a circuit for determining the cross product of the I and Q components with the channel estimate in a QPSK modulation communication system is disclosed in U.S. Pat. No. 5,506,865, entitled, “PILOT CARRIER DOT PRODUCT CIRCUIT,” assigned to the assignee of the present invention, the disclosure of which is incorporated by reference herein.
There has been an increasing demand for wireless communications systems to be able to transmit digital information at high rates. One method for sending high rate digital data from a remote station to a central base station is to allow the remote station to send the data using spread spectrum techniques of CDMA. One method that is proposed is to allow the remote station to transmit its information using a small set of orthogonal channels, this method is described in detail in U.S. Pat. No. 6,396,804, entitled,“HIGH DATA RATE CDMA WIRELESS COMMUNICATION SYSTEM,” assigned to the assignee of the present invention and incorporated by reference herein.
FIG. 1 illustrates a conventional system for generating closed loop power control commands. A signal is received at an antenna and provided to receiver (RCVR) 100. Receiver 100 down converts, amplifies and filters the received signal and provides the received signal to demodulator 102. Demodulator 102 demodulates the received signal. Within demodulator 102, is a channel estimate generator (not shown), which estimates the channel characteristics based on a transmitted signal with values known to both the transmitter and the receiver, referred to herein as the pilot signal. The pilot signal is demodulated and the phase ambiguities in the received signal are resolved by taking the dot product of the received signal and the pilot signal channel estimate. The demodulated signal is typically provided to a deinterleaver, which reorders the demodulated symbols in accordance with a predetermined reordering format.
The reordered symbols are provided to decoded 106. The decoded symbols are then optionally provided to a cyclic redundancy check (CRC) bit check element 107. CRC check element 107 locally generates a set of CRC bits from the decoded data and compares those locally generated bits with the estimated received CRC bits. CRC check element 107 provides a signal indicative of the checking of the CRC bits to control processor 110. In addition, decoder 106 may provide other quality metrics such as Yamamoto metric or symbol error rate to control processor 110. In response, control processor 110 outputs either the decoded frame of data or a signal indicative of the erasure of a frame.
In any communication system there is a nominal performance rate. In conventional systems, the performance is determined based upon the frame error rate of the received signal. The frame error rate depends on the average received signal to noise ratio of the received signal or other quality metric related to the received signal. When the frame error rate is less than the target frame error rate, the power control set point is decreased. Conversely, when the frame error rate is greater than the target frame error rate, the set point is increased. In one method for adjusting the signal to noise ratio threshold, the set point is increased by a relatively large amount, for example 1 dB, whenever a frame erasure is detected. Conversely, the signal to nose ratio threshold is decreased by 0.01 dB whenever a frame is properly decoded. Control processor 110 provides the set point to comparator (COMP) 112. In a pilot assisted coherent communication system, the signal to noise ratio is estimated based on the pilot signal. An exemplary method for estimating the signal to noise ratio based on the pilot signal is disclosed in U.S. Pat. No. 5,903,554, filed Sep. 27, 1996, entitled, “METHOD AND APPARATUS FOR MEASURING LINK QUALITY IN A SPREAD SPECTRUM COMMUNICATION SYSTEM,” assigned to the assignee of the present invention and incorporated by reference herein.
The demodulated signal from demodulator 102 is provided to signal to noise ratio calculator (SNR CALC) 108. Signal to noise ratio calculator 108 computes the signal energy based on the energy of the demodulated symbols. In addition, a signal indicative of the received in band energy is provided to signal to noise ratio calculator 108. Signal to noise ratio calculator 108 generates an estimate of the signal to noise ratio of the received signal and provides this estimate to comparator 112.
In comparator 112, the estimated signal to noise ratio is compared with the power control loop set point provided by control processor 110. A signal indicative of the result of the comparison is provided to power control bit generator 114. If the estimated SNR is less than the set point, then power control bit generator 114 provides a message requesting that the transmitting device increase the energy of its transmissions. If the estimated SNR is greater than the set point, then power control bit generator 114 provides a message requesting that the transmitting device decrease the energy of its transmissions.
The power control message which is a single bit message requesting the transmitting device to increase or decrease its transmission energy by a predetermined amount is provided to puncturing element 118. Puncturing element 118 receives modulated traffic data from traffic modulator 120 and punctures the power control message into the traffic data in a predetermined fashion. The traffic data including the power control data is then upconverted, filtered and amplified for transmission to the transmitting device. In response the power control messages, the transmitting device (not shown) increases or decreases the energy of its transmissions in a predetermined fashion.